Query Auto-Completion (QAC) is a popular feature of web search engines that
aims to assist users to formulate queries faster and avoid spelling mistakes by
presenting them with possible completions as soon as they start typing. However,
despite the wide adoption of auto-completion in search systems, there is little
published on how users interact with such services.

In this paper, we
present the first large-scale study of user interactions with auto-completion
based on query logs of Bing, a commercial search engine. Our results confirm that
lower-ranked auto-completion suggestions receive substantially lower engagement
than those ranked higher. We also observe that users are most likely to engage
with auto-completion after typing about half of the query, and in particular at
word boundaries. Interestingly, we also noticed that the likelihood of using
auto-completion varies with the distance of query characters on the
keyboard.

Overall, we believe that the results reported in our study
provide valuable insights for understanding user engagement with auto-completion,
and are likely to inform the design of more effective QAC systems.